A common approach to measure the rate of fluid flow is a turbine flow meter in which a turbine is caused to rotate in a passage through which the fluid is directed. The rate of rotation of the turbine is directly proportional to the velocity of fluid flow, and by measuring such rate of rotation, a flow rate indication may be obtained.
One application for fluid flow meters which provide a measure of the mass rate of fluid flow is in automotive air flow sensors for fuel control systems. In these applications, the rotating turbine design has the disadvantage of having relatively high inertia such that the response time of such rotating turbine flow meters is inadequate, particularly in view of the great range of the rates of air flow encountered in such applications.
One variation of the turbine type flow meter is the "stalled" turbine flow meter in which a rotor is provided with turbine blades and is disposed in the fluid flow passage and mounted for only a limited degree of angular displacement. The turbine rotor being restrained, reactive forces are created by the imposition of a change of angular momentum on the fluid flowing across the turbine blades. The reactive force is measured by a transducer in order to produce a signal corresponding to the fluid flow.
The restraining of the turbine allows a relatively rapid response time since the inertia is much less than the turbine rotating.
In co-pending U.S. Pat. No. 4,186,602, issued Feb. 5, 1980, assigned to the assignee of the present application, there is disclosed such a mass rate flow meter. In this design, temperature and pressure sensors are provided in addition to the reactive force measuring sensor so as to provide a signal corresponding to the fluid density, such that the mass rate of fluid flow can be computed from the flow rate and density signals.
In this design, the turbine blades are configured to receive substantially the entire fluid flow thereacross such as to provide a high degree of accuracy.
However, a characteristic of fluid flow and the forces generated by the change in momentum of the fluid is that the reactive force generated by the air flow corresponds to the square of second power of the air flow velocity. This nonlinear relationship produces relatively poor response at very low air flow rates and requires flow meters having relatively great ranges since the rate of air flow varies drastically, i.e., a 20:1 air flow range requires a force sensing transducer with a 400:1 output signal range.
A transducer of this range with good resolution in the lower force ranges is technically very difficult to achieve.
A poor response at low air flow rates, on the other hand, is very disadvantageous since the fuel control is more critical due to emission requirements, i.e., the management of fuel to the engine must be carefully controlled at low air flow rates in order to maintain emissions within desired or legal limits.
Accordingly, it is an object of the present invention to provide a turbine type flow meter in which the relationship between the reactive force acting on the stalled turbine and the rate of air flow is more linear with respect to each other such that for increasing air flow, the reactive forces are increased only moderately with increasing air flow.
It is another object of the present invention to provide a stalled turbine type flow meter in which the range of reactive forces which must be sensed by the force transducer is greatly reduced such as to enable the use of relatively low cost transducers for measuring the reactive forces generated by the stalled turbine.